How to use generateFunc in server.cache() in Hapi.js?

Question

How to use generateFunc, as seen below?

server.cache({
    expiresIn: 1000*60*60,
    segment: 'test',
    generateFunc: function(key,next){}
});

I know generateFunc is used to generate a new cache item if one is not found in the cache when calling get() - straight from the api docs. However, I am still not sure how to use get() with generateFunc()

• if I use get() with generateFunc(), is get's callback generateFunc's callback (named next())?
• if I have expiresIn defined in server.cache, do I have to define ttl in get's callback and generateFunc's callback?
• do I still use set method inside generateFunc's callback (next()) to cache the item?

• if I'm using the following in a class object, how can I reference the class variables from the constructor from within generateFunc?

   class Test { constructor(){ this.testVar = 'hello'; } register(server) { this.server = server; this.testCache = server.cache({ expiresIn: 1000*60*60, segment: 'test', generateFunc: function(key,next){} }); } } 

I don't understand the flow. I need a simple example outline with explanations, but I cannot find any good resources.

From the Documentation:

generateFunc (id, next) - a function used to generate a new cache item if one is not found in the cache when calling get().

• id - the id string or object provided to the get() method.
• next - the method called when the new item is returned with the signature function(err, value, ttl) where:
  • err - an error condition.
  • value - the new value generated.
  • ttl - the cache ttl value in milliseconds. Set to 0 to skip storing in the cache. Defaults to the cache global policy.

get (id, callback), where:

• id - the unique item identifier.

• callback , function(err, value, cached, report), where:

  • err - any errors encountered.
  • value - the fetched or generated value.

  • cached - null if a valid item was not found in the cache, or an object with the following keys:

  • • item - the cached value.
  • • stored - the timestamp when the item was stored in the cache.
  • • ttl - the cache ttl value for the record.
  • • isStale - true if the item is stale.

set (id, value, ttl, callback), where:

• id - the unique item identifier
• value - the string or object value to be stored.
• ttl - a time-to-live value in milliseconds after which the item is automatically removed from the cache (or is marked invalid).
• callback - a function with the signature function(err).

Answer 1

Answers on your questions may be found in the docs, sources and through experimentation: http://hapijs.com/tutorials/caching
https://github.com/hapijs/catbox

1. You call generateFunc 's callback, then catbox handles (err, value) , then it calls get 's callback.
2. ttl "Defaults to the cache global policy" . I guess expiresIn is that policy.
3. Don't call set from generateFunc , pass value to catbox via next and it will store it for you.
4. There are several ways of accessing this . I haven't tested the code:

class Test {
  constructor(){
    this.testVar = 'hello';
  }

  register(server) {
    this.server = server;
    const self = this;
    this.testCache = server.cache({
      expiresIn: 1000*60*60,    
      segment: 'test',
      // Old trick
      generateFunc: function(key,next){ console.log(self.testVar); },
      // or new trick
      generateFunc: (key,next) => console.log(this.testVar) // Read about arrow functions
      // or
      generateFunc: this._cacheGenerateFunc.bind(this),

    });
  }

  _cacheGenerateFunc(id, next) {/* Your code */}

}